In recent years there has been an increased demand for integrated circuit chips that can operate with lower power supply voltage signals. Previously, many integrated circuits used to operate with DC power supply voltage signals, V.sub.DD, in the range of about 5 volts. Recently, some manufacturers have developed integrated circuits that operate with DC power supply voltage signals, V.sub.DD, in the range of 3 volts.
Most of the logic circuits, digital circuits, and some of the analog components of integrated circuits, work well with lower voltage signals, such as about 3 volts. However, there are many switching transistors employed in such integrated circuits which are intended to switch reference voltage signals having amplitudes that range between V.sub.DD and ground. Typically, such switching devices operate in response to an applied clock signal. For lower DC power voltage signals, such as 3 volts, these switching transistors may need to switch signals even lower than 3 volts. As a result, the switch signal applied to the gate source of a switching transistor may not be sufficient to activate the switches. For example, in order to turn a MOSFET switch "on" or "off", it may be required to apply a clock signal to the gate of the transistor having a magnitude larger than the lower level power supply voltage signals, such as about 3 volts.
Typically, in prior art devices employing a low voltage DC power supply, a level shifter or a fixed voltage boost arrangement is used to generate the required clock signals for operating the switches. Therefore, a voltage level shifter or a booster circuit may provide a clock signal that has a magnitude higher than the DC power source, sufficient to operate the switching devices reliably.
Recently, there has been a demand for integrated circuits that can operate with either low or high voltage DC power supplies, such as 3 volts and 5 volts. A shortcoming with prior art fixed level shifters or boost circuits is that they are not appropriate for integrated circuits that are intended to operate with either low or high voltage DC power supplies. This follows because a fixed level shifter or boost circuit that generates a clock signal, appropriately shifts the magnitude of a lower supply voltage signal to an appropriate clock signal level. However, the same level shifter may shift the magnitude of a higher supply voltage signal to a clock signal level that is sufficiently high to damage the switching devices.
Furthermore, in many battery operated applications, the voltage level of the battery diminishes with use and time. Therefore, when the battery is fresh, the integrated circuit powered by the battery works appropriately. However, as the life of the battery diminishes, the clock signal applied to the switching circuits may not attain a sufficient level capable of operating the switches. Again a fixed level shifter or a boost circuit may not be appropriate in this situation. When the battery is fresh, the fixed level shifter may provide a clock signal level that is sufficiently high to cause damage to the switching devices.
Thus, there is a need for a variable level shifter or a boost circuit which can operate with a variable DC power supply that generates either a low or high DC power signal, and can generate an appropriate boost in response to the DC power supply voltage level.